There is a range of options to reduce the impact of aviation emissions,
including changes in aircraft and engine technology, fuel, operational practices,
and regulatory and economic measures. These could be implemented either singly
or in combination by the public and/or private sector. Substantial aircraft
and engine technology advances and the air traffic management improvements described
in this report are already incorporated in the aircraft emissions scenarios
used for climate change calculations. Other operational measures, which have
the potential to reduce emissions, and alternative fuels were not assumed in
the scenarios. Further technology advances have the potential to provide additional
fuel and emissions reductions. In practice, some of the improvements are expected
to take place for commercial reasons. The timing and scope of regulatory, economic,
and other options may affect the introduction of improvements and may affect
demand for air transport. Mitigation options for water vapor and cloudiness
have not been fully addressed.

Safety of operation, operational and environmental performance, and costs are
dominant considerations for the aviation industry when assessing any new aircraft
purchase or potential engineering or operational changes. The typical life expectancy
of an aircraft is 25 to 35 years. These factors have to be taken into account
when assessing the rate at which technology advances and policy options related
to technology can reduce aviation emissions.

6.1. Aircraft and Engine Technology Options

Technology advances have substantially reduced most emissions per passenger-km.
However, there is potential for further improvements. Any technological change
may involve a balance among a range of environmental impacts.

Subsonic aircraft being produced today are about 70% more fuel efficient per
passenger-km than 40 years ago. The majority of this gain has been achieved
through engine improvements and the remainder from airframe design improvement.
A 20% improvement in fuel efficiency is projected by 2015 and a 40 to 50% improvement
by 2050 relative to aircraft produced today. The 2050 scenarios developed for
this report already incorporate these fuel efficiency gains when estimating
fuel use and emissions. Engine efficiency improvements reduce the specific fuel
consumption and most types of emissions; however, contrails may increase and,
without advances in combuster technology, NOx emissions may also increase.

Future engine and airframe design involves a complex decision-making process
and a balance of considerations among many factors (e.g., carbon dioxide emissions,
NOx emissions at ground level, NOx emissions at altitude, water vapor emissions,
contrail/cirrus production, and noise). These aspects have not been adequately
characterized or quantified in this report.

Internationally, substantial engine research programs are in progress, with
goals to reduce Landing and Take-off cycle (LTO) emissions of NOx by up to 70%
from today's regulatory standards, while also improving engine fuel consumption
by 8 to 10%, over the most recently produced engines, by about 2010. Reduction
of NOx emissions would also be achieved at cruise altitude, though not necessarily
by the same proportion as for LTO. Assuming that the goals can be achieved,
the transfer of this technology to significant numbers of newly produced aircraft
will take longer-typically a decade. Research programs addressing NOx emissions
from supersonic aircraft are also in progress.

6.2. Fuel Options

There would not appear to be any practical alternatives to kerosene-based
fuels for commercial jet aircraft for the next several decades. Reducing sulfur
content of kerosene will reduce SOxO emissions and sulfate particle formation.

Jet aircraft require fuel with a high energy density, especially for long-haul
flights. Other fuel options, such as hydrogen, may be viable in the long term,
but would require new aircraft designs and new infrastructure for supply. Hydrogen
fuel would eliminate emissions of carbon dioxide from aircraft, but would increase
those of water vapor. The overall environmental impacts and the environmental
sustainability of the production and use of hydrogen or any other alternative
fuels have not been determined.

The formation of sulfate particles from aircraft emissions, which depends on
engine and plume characteristics, is reduced as fuel sulfur content decreases.
While technology exists to remove virtually all sulfur from fuel, its removal
results in a reduction in lubricity.

6.3. Operational Options

Improvements in air traffic management (ATM) and other operational procedures
could reduce aviation fuel burn by between 8 and 18%. The large majority (6
to 12%) of these reductions comes from ATM improvements which it is anticipated
will be fully implemented in the next 20 years. All engine emissions will be
reduced as a consequence. In all aviation emission scenarios considered in this
report the reductions from ATM improvements have already been taken into account.
The rate of introduction of improved ATM will depend on the implementation of
the essential institutional arrangements at an international level.

Air traffic management systems are used for the guidance, separation, coordination,
and control of aircraft movements. Existing national and international air traffic
management systems have limitations which result, for example, in holding (aircraft
flying in a fixed pattern waiting for permission to land), inefficient routings,
and sub-optimal flight profiles. These limitations result in excess fuel burn
and consequently excess emissions.

For the current aircraft fleet and operations, addressing the above-mentioned
limitations in air traffic management systems could reduce fuel burned in the
range of 6 to 12%. It is anticipated that the improvement needed for these fuel
burn reductions will be fully implemented in the next 20 years, provided that
the necessary institutional and regulatory arrangements have been put in place
in time. The scenarios developed in this report assume the timely implementation
of these ATM improvements, when estimating fuel use.

Other operational measures to reduce the amount of fuel burned per passenger-km
include increasing load factors (carrying more passengers or freight on a given
aircraft), eliminating non-essential weight, optimizing aircraft speed, limiting
the use of auxiliary power (e.g., for heating, ventilation), and reducing taxiing.
The potential improvements in these operational measures could reduce fuel burned,
and emissions, in the range 2 to 6%.

Improved operational efficiency may result in attracting additional air traffic,
although no studies providing evidence on the existence of this effect have
been identified.